With the exception of L-DOPA pharmacological administration for Parkinson's disease, neurodegenerative diseases in general lack effective treatment. Previous studies of neurodegenerative diseases suggest that symptoms arise secondary to defects in local neural circuitry and cannot be treated effectively with systemic drug delivery. Consequently, alternative treatments for neurodegenerative diseases have emerged. Such treatments include transplantation of genetically engineered cells (See e.g., Breakefield, X. O. et al. (1989) Neurobiol. Aging 10:647-648; Gage, F. H. et al. (1987) Neuroscience 23:795-807; Horellou P. et al. (1990) Eur. J. Neurosci. 2:116-119; Rosenberg, M. B. et al. (1988) Science 242:1575-1578; Wolff, J. A. et al. (1989) Proc. Natl. Acad. Sci. USA 86:9011-9014) or fetal cells (See e.g., Bjorklund, A. et al. (1983) Acta. Physiol. Scand. Suppl. 522:1-75; Dunnett, S. B. et al. (1990) in Brain Repair (eds. Bjorklund, A. et al.) Wenner-Gren International Symposium Series 56:335-373 (McMillan Press, London); Isacson, O. et al. (1984) Nature 311;458-460) into the area of neurodegeneration in an effort to reconstitute damaged neural circuits, and to replace lost neurons and neurotransmitter systems.
Engineered cells can be derived from cell lines or grown from recipient host fibroblasts or other cells and then modified to produce and secrete substances following transplantation into a specific site in the brain. Neuroactive substances amenable to this delivery mode include neuropeptides and chemical transmitters. For example, one group of researchers developed a biological system in which genetically engineered nerve growth factor-producing rat fibroblasts, when implanted into the rat striatum prior to infusion of neurotoxins were reported to protect neurons from excitotoxin-induced lesions (Schumacher, J. M. et al. (1991) Neuroscience 45(3):561-570). Another group which transplanted rat fibroblasts genetically modified to produce L-DOPA or dopamine into 6-hydroxydopamine lesions of the nigrostriatal pathway in rats reported that the transplanted fibroblasts reduced behavioral abnormalities in the lesioned rats (Wolff, J. A. et al. (1989) Proc. Natl. Acad. Sci. USA 86:9011-9014). Alternative to genetically engineered cells, cells to be implanted into the brain can be selected because of their intrinsic release of critical compounds, e.g., catecholamines by PC12 cells and nerve growth factor by immortalized hippocampal neurons.
Transplantation of cells engineered to produce and secrete neuroactive substances can be used alone or in combination with transplantation of fetal neural progenitor cells into areas of neurodegeneration in the brain. In order to repair functional connections damaged by neurodegeneration in, for example, the striatum, cells transplanted into the area of striatal neuron loss must re-establish synaptic connectivity with neurons in a number of target structures located a considerable distance from the area of neurodegeneration. Axonal tracing of connections of intrastriatal allografts in rats demonstrate that both afferent and efferent connections are established between graft neurons and host neurons in appropriate areas (Labandeira-Garcia, J. L. et al. (1991) Neuroscience 42:407-426; Liu, F. C. et al. (1990) J. Comp. Neurol. 295:1-14; Wictorin, K. et al. (1988) Neuroscience 27:547-562; Wictorin, K. et al. (1989) Neuroscience 30:297-311; Xu, Z. C. et al. (1991) J. Comp. Neurol. 303:22-34) and host-graft connections have been substantiated by electrophysiological and ultrastructural analysis. Rutherford, A. et al. (1987) Neuroscience Lett. 83:275-281; Xu, Z. C. et al. (1991) J. Comp. Neurol. 303:22-34. However, the extent of efferent connections from striatal allografts is limited, with respect to number of connections (Walker, P. D. et al. (1987) Brain Res. 425:34-44; Wictorin, K. et al. (1989) Neuroscience 30:297-311) and with respect to connections to distant targets (McAllister, J. et al. (1989) Brain Res. 476:345-350; Pritzel et al. (1986); Wictorin, K. et al. (1989) Neuroscience30:297-311; Zhou, H. F. et al. (1989) Brain Res. 504:15-30). There is a need, therefore, for sources of neural progenitor cells and methods of neural transplantation which promote or enhance development of efferent connections from the transplant to the recipient brain tissue and connections with distant recipient brain targets.
In order to replace dopaminergic cells damaged by neurodegeneration in, for example, the substantia nigra, cells transplanted into the area of dopaminergic neuron loss must saturate the striatum with terminals and produce dopamine via a feedback control system. Cells which are engineered to express enzymes which act in the biosynthesis of dopamine are known to constitutively secrete dopamine. Kang, U. J. et al. (1993) J. Neurosci. 13(12):5203-5211. The constitutive secretion of dopamine was reported to be without significant storage capacity in vesicles or regulation at the level of secretion. Kang, U. J. et al. (1993) J. Neurosci. 13(12):5203-5211; See also Fisher, L. J. et al. (1993) Ann. N.Y. Acad. Sci. 695:278-284 (citing constitutive secretion of neurotransmitter as unaddressed issue). Thus, there is also a need for sources of neural progenitor cells which produce neurotransmitters via a feedback control mechanism.